32 NILS ILT. ODHNER, STUDIES ON RECENT CHAMIDAE. 



Ch. pellucida the pericardial tube is »in die Epithelfalten der Innenwand des Aussen- 

 sackes eingehiillt» (Grieser 1913, p. 252, ef. fig. 9). Finally it debouches, thus re- 

 taining its narrow breadth throughout, into the dorsal part of the outer sac, while 

 in the new species it is widened dorsally and passes into the apical end of the outer 

 sac. Further in Ch. 'pellucida the lobes and the lining cells are larger. 



In 1912, as well as on a renewed investigation, I found exactly the same con- 

 ditions as in Ch. pellucida in another sinistral form from the West Indies, which 

 has turned out to be Chama sarda Reeve. 



The differences in the shape of the nephridium on the one hand, as well as 

 the accordance of the organization on the other, are of such importance compared 

 with the few and trifling cases of inversion that they completely upset the opinion 

 which has prevailed hitherto even among anatomists and which is expressed by 

 Pelseneer (1911, p. 58, ef. p. 86) in these words: »Les Chama fixées sur le coté 

 droit présentent un situs inversus par rapport aux Chama gauches. » 



Thus the nephridium, above all other organs, seems to support the same com- 

 ception of the dextral forms as the strueture and the development of the hinge give 

 rise to, namely that these must be separated from Chama and kept apart as con- 

 stituting a distinct genus. It now remains to investigate whether in other species 

 the same essential organization prevails as in the types compared above, and on 

 this more comprehensive inquiry to base our definite conclusions as to the reciprocal 

 relations and systematical arrangement of the Lamellibranchs which constitute the 

 object of the present investigation. 



5. Anatomien] researclies on species of Psendochaina. 



Echinochama arcinella Linné. 



Figs. 28-32. 



Tvvo specimens from Cape Sable, Florida, kindly sent to me by Dr. W. H. 

 Dall, Washington, have been the subject of the following examination. 



Externa] morpliolocry (fig. 28). 



The mantle is thin all över, even on the umbones, which project to an equal 

 level. The free mantle margin is smooth, somewhat wavy due to the shell sculp- 

 ture; the inner mantle folds are, as usual, coalesced, their exteriör surface is minu- 

 tely and irregularly tubercled. The siphonal openings occupy about 2 /& — 1 /s of the 

 whole under margin, the pedal slit is rather small and lies just under the end of 

 the anterior adduetor, occupying a space of about 2 /-> of itslength; the anal siphonal 

 opening sends a large fold inward, and the inspiratory opening has the margins ele- 

 vated to a distinct siphon, the lenght of which may attain to about 1 7« times the 



